My brother's Windows (Vista) PC has recently become incredibly slow, to the point where you can't really do anything without waiting at least 15 minutes. I think it's got a fair amount of space left on the hard drive, so I don't really know what's going on.

I thought if I installed Linux on his computer then he would at least be able to connect to the Internet and use his computer quickly. The thing is, the Windows on his computer is so slow that I want to spend as little time as possible using Windows in order to install Linux. I imagine you'd have to connect to the Internet to download the distribution and things, but I'd otherwise like to use the existing OS as little as possible when installing the new one.

I have got a working computer (the one I'm using to write this question) if that helps.